The invention relates to an apparatus for separating dc current and ac current components of a composite signal. It relates in particular to a circuit arrangement for separating dc and ac current components of a composite signal in a subscriber line station.
The necessity of separating signals arises in general in measurement circuits.
In this connection, a special application example outside this range should be mentioned, namely the conditions of modern subscriber line circuits for connecting subscriber lines to digital time division multiplexing telephone systems.
For handling a composite signal which occurs in subscriber lines in the form of a loop current superposed by voice signals, subscriber line circuits usually have to derive loop status indications from the dc current portion of the loop current and to simulate the correct ac current impedance for an optimal signal matching of the voice signal to be transmitted.
In the interest of an optimal integration capacity, modern subscriber line circuits are subdivided into different hardware modules, of which a high voltage power module and a low voltage module are configured as integrated circuits, while a safety element module and a test module are assembled with discrete components.
While such tasks as call recognition, voice transmission, the establishing of line symmetry, subscriber line drive and the separation of longitudinal current components are performed by the high voltage power module, the low voltage module is used for feeding charge pulses, for realizing different monitoring functions, for setting the two wire impedance, and for detecting the loop status.
When using this type of configuration a sum signal forwarded by the power module and comprising dc and ac components must be separated into its constituents to obtain control variables with which the requested supply characteristics in form of a respective dc current resistance and a requested impedance can be set.
In principle, this kind of signal separation can be obtained with the use of RC combinations. However, in the frequency range of the voice band (300 Hz to 3400 Hz), which is of interest here, capacitance values of approximately 100 .mu.F are required. Moreover, the dc current signals occur with dc components of either polarity, so that nonpolar capacitors must be used. These requirements are met only by foil capacitors, which are both bulky and expensive.
It is therefore an object of the invention to provide an apparatus which separates as completely as possible the dc and ac current components of a composite signal.
It is another object of the invention to provide an apparatus for separating the dc and ac current components of a composite signal in a compact and cost-effective manner.
Yet another object of the invention is to provide an apparatus for separating the dc and ac current components of a composite signal in the voice band.
A further object of the invention is to provide an apparatus which separates the dc and ac current components of a composite signal such that even ac current components of high amplitudes are not distorted.
Yet a further object is, in general, to improve on known apparatus of this type.